The present invention relates to a disk player for reproducing recorded audio data from a digital audio disk such as CD (Compact Disc) or MD (Mini Disc) (hereinafter referred to as disk), and more particularly to a disk player wherein the audio data reproduced from a disk are once stored in a bulk memory of a large capacity and then are read out therefrom to be outputted. And the invention further relates to a servo system for use in such a disk player.
In disk players inclusive of CD players, there is known a shock-proof type of which construction is so contrived that, when any track jump is caused during reproduction of recorded data due to some external disturbance such as shock, the continuity of the reproduced data is maintained to prevent occurrence of any sound skip.
Here, "track jump" signifies a phenomenon that a data-reading light spot of an optical pickup for reading out the recorded data while following up a record track (pit string) on a disk jumps the record track.
In an ordinary CD player, a data rate for reproduction of the recorded data from a disk (hereinafter referred to as reproduction data rate) is equal to a data rate for outputting the audio data (hereinafter referred to as output data rate).
However, in any conventional shock-proof type CD player, a disk is driven at a higher rotation speed which is twice the normal speed in an ordinary CD player, so that the data are read at a reproduction data rate higher than that in the ordinary CD player and, while the read data thus obtained are stored temporarily in a bulk memory of a large capacity, the stored data are read from the bulk memory at the same output data rate as in the ordinary CD player.
And when there occurs a track jump during data reproduction, the data-reading light spot of the optical pickup is returned to the position immediately before occurrence of such a track jump, and the data reproduction is resumed from that position while the reproduced audio data obtained after resumption of the reproduction are linked, by utilizing the stored data in the bulk memory, to the preceding audio data reproduced immediately before occurrence of the track jump, whereby the continuity of the reproduced audio data is maintained to consequently prevent generation of any sound skip.
Furthermore, in the known construction where the normal rotation speed of a disk is twice the velocity in an ordinary CD player, each of servo systems for focus, tracking, sled and spindle control is kept in operation, and when the data quantity stored in the bulk memory has exceeded a preset value, the result of monitoring the stored data quantity is detected as an overflow, and then a reverse jump of, e.g., one track is repeated in a standby state.
As described, in the conventional shock-proof type CD player of the above construction, a disk is normally driven to rotate at a high speed and each servo system is kept in operation, so that the power consumption is naturally increased, and this problem is considered to be a disadvantage particularly in a portable CD player where low power consumption is requisite.
There is known another shock-proof CD player proposed by the present applicant (as disclosed in Japanese Patent Laid-open No. Hei 4 (1992)-268249), wherein a disk rotation rate is changed to a double speed (high speed mode) only in response to occurrence of a track jump caused by some external disturbance such as shock, and storage of data into a bulk memory of a large capacity is performed fast, but the disk is driven at a regular speed (low speed mode) in any other case to consequently realize a reduction of the power consumption.
In the above shock-proof CD player, when a servo circuit for controlling the rotation of a disk-driving spindle motor switches the driving operation from a low speed mode to a high speed mode or vice versa, the frequency division ratio of a clock signal used as a reference for speed control is gradually changed, and pitch control is executed with respect to the period of such clock signal to thereby produce a frequency error signal, and the rotation speed of the spindle motor is controlled in accordance with the frequency error signal thus obtained.
As mentioned, in the conventional rotation servo circuit which executes pitch control with respect to the period of a clock signal serving as a reference for speed control, ideal speed control is achievable on the one hand, but due to the necessity of employing a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) circuit, there exist some problems including that the circuit configuration is complicated and the production cost is raised.
The spindle (rotation) servo system adopted in the above known example has a rotation servo circuit which comprises a rough servo system for pulling the disk rotation speed approximately into a desired precision range and a phase lock servo system for attaining a higher precision after the control action of the rough servo system. This phase lock servo system has a secondary loop structure which consists of a phase loop for executing a phase locking control in accordance with the phase difference between a reproduced sync signal and a reference sync signal, and a speed loop for measuring the period of a reproduced frame sync signal obtained from the disk and equalizing the frequency thereof to a reference frequency of 7.35 kHz. Meanwhile the rough servo system has a primary loop structure which extracts the lowest-frequency component of the reproduced signal and equalizes the frequency of the extracted component to the reference frequency.
In the control action of the rotation servo circuit, first the disk rotation speed is pulled approximately into the desired precision range by the rough servo system.
As a result, the phase-locked loop (PLL) of the phase lock servo system is locked to attain a state where data is reproducible from the disk. When the PLL is thus locked, the primary-loop rough servo system is switched to the secondary-loop phase lock servo system.
However, in the conventional rotation servo circuit of the above-described construction, there arises a problem that a phase-locked state is not held at the moment the rough servo system is switched over to the phase lock servo system, although the disk rotation speed is substantially kept stable. And in an exemplary case where the frame jitter margin is set to .+-.27 frames and the existing deviation is a maximum of 27 frames, it follows that such state is forcibly pulled into synchronism to consequently cause a disorder in the rotation speed at this moment, whereby the PLL is induced to be out of phase.
If the PLL is rendered out of phase in this manner, it is impossible to read each subcode recorded as an index for the data on a disk. Therefore, an operation of reading the subcode becomes possible posterior to the lapse of a fixed time which is required until the disk rotation speed is stabilized after the rough servo system is switched to the phase lock servo system.
In a seek for replacing an optical pickup to a desired address position (target address position) on a disk, as shown in FIG. 1, there are performed a series of operations of first switching the phase lock servo system to the rough servo system, then replacing the pickup in the radial direction of the disk by a distance corresponding to a required number of tracks, subsequently switching the rough servo system to the phase lock servo system after the disk rotation speed has reached approximately to its regular linear velocity, and reading the subcode from the disk. The above operations are repeated until the pickup has arrived at the target address position.
Therefore, if the disk rotation speed is disordered at the moment of switching the rough servo system to the phase lock servo system as described, a speed disorder is caused at each time of the repeated operations of switching from the rough servo system to the phase lock servo system during the seek, whereby the time required for reading out the subcode is prolonged to eventually fail in shortening the seek time.
Particularly in reproduction of data from a CD-ROM or the like where a fast seek is necessary, any disorder of the disk rotation speed causes a waste of time in reading out the subcode, hence raising an important problem in realizing fast seek.